With CUE this can't be changed and is one of the drawbacks of using a Group or GDM Mailbox :(
General delivery mailbox (GDM) This mailbox is assigned to a group of subscribers. All members in the group have access to the mailbox. When a caller leaves a message in this mailbox, **no MWI is turned on**. Instead, when a member logs in to the personal mailbox, the mailbox menu allows the member to access the messages in each GDM to which the member belongs. Only one person can access the GDM at a time. After the first person saves or deletes a message in the GDM, the message is no longer played as "new" for any subsequent members.
I can't test this right now, but I think it should work but will only give the Envelope icon not MWI Lamp because 8900 is not the Primary number on the phones. Because this is the Main Number you may want to make it the Primary (Button 1 ) on at least one of the two Reception phones just because of this parameter.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...